1. Field of the Invention
The present invention relates to a gas discharge lamp lighting device used as a light source such as a lamp for use with a vehicle display of a projection type.
2. Description of the Prior Art
FIG. 17 is a schematic circuit diagram showing the structure of a prior art gas discharge lamp lighting device as disclosed in Japanese patent application publication (TOKKAIHEI) No. 12-82592. In FIG. 17, reference numeral 1 denotes a direct-current power supply such as a battery, and numeral 2 denotes a DC/DC converter for adjusting electric power supplied from the direct-current power supply 1 and for outputting the adjusted electric power supply. The DC/DC converter 2 includes a transformer 2a, an FET (field effect transistor) 2a, and a diode 2c. Reference numeral 3 denotes a ground, numeral 4 denotes a shunt resistor used for detection of an electric current IL flowing through a gas discharge lamp 12, and numeral 50 denotes an H-type full bridge circuit (referred to as xe2x80x9cH-bridgexe2x80x9d hereafter) that consists of a plurality of FETs 50a to 50d, and that converts the DC power adjusted by the DC/DC converter 2 to AC power. The gas discharge lamp 12 is driven by the AC power to which the DC power is converted by the H-bridge 50.
Furthermore, reference numeral 13 denotes an interface circuit (referred to as I/F from here on) that accepts a lamp voltage VL from a cathode-side output of the DC/DC converter 2, and that accepts a lamp electric current IL from an H-bridge side end of the shunt resistor 4, and numeral 14 denotes a control circuit for controlling the FET 2b of the DC/DC converter 2 based on the lamp voltage VL and the lamp electric current IL, which are detected successively by way of the I/F 13, and a predetermined circuit impedance so that the electric power supplied to the gas discharge lamp 12 reaches a predetermined value.
In operation, when causing the gas discharge lamp 12 to start to light, the DC/DC converter 2 adjusts the DC electric power supplied from the direct-current power supply 1 and outputs the adjusted. DC electric power, and the H-bridge 50 then converts the adjusted DC electric power from the DC/DC converter 2 to AC power so as to drive the lamp 12. The lamp voltage VL detected at the cathode side of the output of the DC/DC converter 2 is raised up to xe2x88x92400V as shown in FIG. 18. The gas discharge lamp 12 is made to light up after the lamp voltage VL is further increased up to about 20 kV at its peak, and, after that, the lamp is put in a stable lighting status at xe2x88x9290V. In the meanwhile, the DC/DC converter 2 is controlled by the control circuit 14. The control circuit 14 controls the FET 2b of the DC/DC converter 2 based on the lamp voltage VL and the lamp electric current IL, which are detected successively by way of the I/F 13, so that the electric power supplied to the lamp 12 reaches a predetermined value.
After the lamp 12 is made to light up, the control circuit 14 applies the AC voltage to the lamp 12 by alternately Switching between a switching mode of turning on the FETs 50a and 50d of the H-bridge 50 and turning off the other FETs 50b and 50c of the H-bridge 50, and another switching mode of turning off the FETs 50a and 50d of the H-bridge 50 and turning on the other FETs 50b and 50c of the H-bridge 50.
By the way, it is preferable that the electric power supplied to the lamp 12 put in the stable lighting status is 34 watts. The control circuit 14 does not simply control the electric power supplied to the lamp 12 based on only the lamp voltage VL and the lamp electric current IL so that it reaches 34 watts. By estimating the on-resistance of each of the FETs 50a to 50d of the H-bridge 50 in advance so as to make an estimate of the circuit impedance, the control circuit 14 performs the control operation based on the lamp voltage VL, the lamp electric current IL, and the circuit impedance estimated beforehand so that the electric power supplied to the lamp 12 reaches 34 watts even if there is a power loss due to the on-resistance of each of the FETs 50a to 50d of the H-bridge 50.
In the prior art gas discharge lamp lighting device constructed as above, since a high voltage of 400V or less is applied to the H-bridge 50, each of the FETs which constitute the H-bridge 50 has to withstand a high voltage of 400V. The unit price of FETs having such a high voltage breakdown is high, and the above-mentioned H-bridge of the prior art gas discharge lamp lighting device uses as much as four FETs with such a high unit price. The inverter circuit structure, by using the H-bridge as mentioned above, therefore obstructs downsizing of the gas discharge lamp lighting device and a reduction in the cost of the device. A decrease in the number of FETs included in the H-bridge 50 and a reduction in the voltage applied to the H-bridge are therefore challenges for the prior art gas discharge lamp lighting device.
On the other hand, Japanese patent application publication (TOKKAIHEI) No. 8-195288 discloses another prior art gas discharge lamp lighting device for driving a gas discharge lamp by applying AC power to the lamp using two semiconductor switching elements (transistors) and a capacitor without the use of an H-bridge like the above-mentioned H-bridge of FIG. 18. FIG. 19 is a schematic circuit diagram showing the structure of the other prior art gas discharge lamp lighting device. In FIG. 19, reference numeral 61 denotes a gas discharge lamp, numeral 62 denotes a lighting device, numeral 63 denotes a battery, numeral 64 denotes a transistor, numeral 65 denotes a diode, numeral 66 denotes a choke coil, numeral 67 denotes a capacitor, numeral 68 denotes a control circuit, numeral 69 denotes a step-down chopper circuit, numeral 70 denotes a direct-current power supply, numerals 71 and 72 denote transistors, numeral 73 denotes a capacitor, numeral 74 denotes an inverter circuit, numeral 75 denotes an inductor, numeral 76 denotes a start circuit, numeral 77 denotes a lamp voltage detector, numeral 78 denotes a driving circuit, numeral 79 denotes a control unit, numeral 80 denotes a lamp electric current detector, and numeral 81 denotes a detector for detecting electric power applied to the lamp 61.
In operation, electric power from the battery 63 within the direct-current power supply 70 is adjusted by the step-down chopper circuit 69, and is furnished. to the inverter circuit 74. The transistor 71 is turned on and the transistor 72 is turned off in the inverter circuit 74. As a result, an electric current flows from the step-down chopper circuit 69 into the discharge lamp 61 by way of the capacitor 73, and the electric current is then supplied to the gas discharge lamp 61 while the capacitor 73 is charged up. By turning off the transistor 71 and turning on the transistor 72, the electric charge stored in the capacitor 73 is then made to flow to the gas discharge lamp 61 as an electric current flowing in the opposite direction to that of the above-mentioned electric current flowing from the step-down chopper circuit 69 to the gas discharge lamp 61. Thus, by alternately switching between a state in which the capacitor 73 is charged up with the supply of the electric current from the transistor 71 to the gas discharge lamp 61 and another state in which the electric current is supplied from the capacitor 73 to the lamp 61 by turning off the transistor 71 and turning on the transistor 72, an AC current is made to flow into the gas discharge lamp 61.
A problem with the prior art gas discharge lamp lighting device is that when switched into the phase in which the electric current is made to flow from the capacitor 73 to the gas discharge lamp 61 and the polarity of the current flowing through the gas discharge lamp is then reversed, the AC gas discharge can be extinguished and stable lighting cannot be implemented if a voltage higher than required to maintain the AC gas discharge cannot be applied from the capacitor 73 to the gas discharge lamp 61.
Furthermore, the amount of electric charge supplied to the gas discharge lamp 61 changes depending on the length of the time period during which the electric current is supplied to the gas discharge lamp 61 while the capacitor 73 is charged up, and the lamp voltage required to maintain the AC gas discharge changes according to the amount of electric charge supplied to the gas discharge lamp 61. Another problem is thus that when switched from the DC phase to the AC phase, the AC gas discharge can be extinguished easily according to the change in the lamp voltage.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a gas discharge lamp lighting device capable of easily lighting a gas discharge lamp, and preventing the gas discharge lamp from being extinguished when causing the lamp to change from an electrode heating state in which both electrodes of the lamp is heated to an AC discharging state in which an AC current flows through the lamp, and the polarity of the current flowing through the lamp is then reversed, and at the beginning of the AC discharging state, by changing the amount of energy to be supplied to the gas discharge lamp placed in the electrode heating state according to a lamp voltage across the lamp.
In accordance with an aspect of the present invention, there is provided a gas discharge lamp lighting device comprising: an electric power adjusting unit for adjusting electric power supplied from a power supply so as to generate and output a DC voltage; a gas discharge lamp driving unit electrically connected to the electric power adjusting unit, for converting the DC voltage from the electric power adjusting unit to an AC voltage to be supplied to a gas discharge lamp; and a control unit for bringing the gas discharge lamp to an electrode heating state in which both electrodes of the gas discharge lamp are heated after supplying the AC voltage to the gas discharge lamp, for controlling an amount of energy to be supplied to the gas discharge lamp placed in the electrode heating state according to a voltage across the gas discharge lamp, and for bringing the gas discharge lamp to an AC discharging state in which an AC current flows through the gas discharge lamp after the amount of energy has been supplied to the gas discharge lamp.
In accordance with a preferred embodiment of the present invention, the gas discharge lamp driving unit can be a switching unit having two output terminals, one of which is connected to one electrode of the gas discharge lamp, and the other of which is connected to another electrode of the gas discharge lamp, for electrically connecting the two output terminals to each other under control of the control unit.
In accordance with another preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises a first capacitor connected between one output terminal of the switching unit and one terminal of the gas discharge lamp which are electrically connected to each other, and a switching element that is connected in parallel to the first capacitor and that is turned on or turned off under control of the control unit. In addition, in order to control the amount of energy to be supplied to the gas discharge lamp in the electrode heating state, the control unit turns on the switching element and supplies a predetermined amount of energy to the gas discharge lamp, and, after that, turns off the switching element to charge up the first capacitor and brings the gas discharge lamp to the AC discharging state when a voltage across the first capacitor reaches a predetermined voltage.
In accordance with another preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises an initial current supplying unit electrically connected between the electric power adjusting unit and the gas discharge lamp driving unit, for supplying an initial current to the gas discharge lamp when the gas discharge lamp starts to discharge. Preferably, the initial current supplying unit can be either a series circuit in which a first resistor and a diode connected in parallel to each other, a capacitor, and a second resistor are connected in series to each other, or a series circuit in which a parallel circuit in which a first resistor and a series circuit having a second resistor and a diode connected in series to each other are connected in parallel to each other, and a capacitor are connected in series to each other. Preferably, the second resistor has a resistance value ranging from 2xcexa9 to 100xcexa9.
In accordance with another preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises an igniter circuit for generating a high voltage based on the DC voltage from the electric power adjusting unit when the gas discharge lamp starts to discharge, and for applying the high voltage to the gas discharge lamp.
In accordance with another preferred embodiment of the present invention, the control unit detects the voltage across the gas discharge lamp when the gas discharge lamp is placed in the AC discharging state so as to control the amount of energy to be supplied to the gas discharge lamp that will be lighted up the next time and that will be placed in the electrode heating state according to the detected voltage. As an alternative, the control unit can detect the voltage across the gas discharge lamp when the gas discharge lamp is placed in the electrode heating state so as to control the amount of energy to be supplied to the gas discharge lamp according to the detected voltage.
In accordance with another preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises a high voltage generating unit for generating and applying a high voltage to the gas discharge lamp so as to prevent the gas discharge lamp from being extinguished when the control unit causes the gas discharge lamp to change from the electrode heating state to the AC discharging state. Preferably, the high voltage generating unit can include a second capacitor connected in parallel to the switching element, and an inductor connected in series to the first capacitor, and a series circuit including the first capacitor and the inductor is connected in parallel to both the switching element and the second capacitor.
In accordance with another aspect of the present invention, there is provided a gas discharge lamp lighting device comprising: an electric power adjusting unit for adjusting electric power supplied from a power supply so as to generate and output a DC voltage; a switching unit electrically connected to the electric power adjusting unit and having two output terminals, one of which is connected to one electrode of a gas discharge lamp, and the other of which is connected to another electrode of the gas discharge lamp, for electrically connecting the two output terminals to each other in response to a control signal applied thereto, and for converting the DC voltage from the electric power adjusting unit to an AC voltage to be supplied to the gas discharge lamp; a first capacitor connected between one output terminal of the switching unit and one terminal of the gas discharge lamp which are electrically connected to each other; a control unit for charging up the first capacitor while supplying the DC voltage to the gas discharge lamp so as to make it discharge, and for stopping the supply of the DC voltage to the gas discharge lamp before bringing the gas discharge lamp to an AC discharging state in which an AC current flows through the gas discharge lamp, and then delivering the control signal to the switching unit so as to supply energy stored in the first capacitor to the gas discharge lamp; and a high voltage generating unit for generating and applying a high voltage to the gas discharge lamp so as to prevent the gas discharge lamp from being extinguished when the control unit performs the control operation so as to supply the energy stored in the first capacitor to the gas discharge lamp.
In accordance with an preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises a switching element that is connected in parallel to the first capacitor and that is turned on or turned off under control of the control unit. In addition, the high voltage generating unit includes a second capacitor connected in parallel to the switching element, and an inductor connected in series to the first capacitor, and a series circuit including the first capacitor and the inductor is connected in parallel to both the switching element and the second capacitor. Preferably, the high voltage generating unit further includes a saturable reactor connected in series to the inductor.
In accordance with another preferred embodiment of the present invention, the high voltage generating unit includes a second capacitor having a capacitance value smaller than that of the first capacitor and having two electrodes electrically connected to both electrodes of the gas discharge lamp, respectively, and an inductor connected in series to the first and second capacitors. Preferably, the high voltage generating unit further includes a saturable reactor connected in series to the inductor.
In accordance with another preferred embodiment of the present invention, the control unit brings the gas discharge lamp to an electrode heating state in which both electrodes of the gas discharge lamp are heated after the gas discharge lamp starts to discharge, controls an amount of energy to be supplied to the gas discharge lamp placed in the electrode heating state according to a voltage across the gas discharge lamp, and brings the gas discharge lamp to an AC discharging state in which an AC current flows through the gas discharge lamp after the controlled amount of energy has been supplied to the gas discharge lamp.
In accordance with another preferred embodiment of the present invention, the gas discharge lamp lighting device further comprises a switching element that is connected in parallel to the first capacitor and that is turned on or turned off under control of the control unit. In addition, in order to control the amount of energy to be supplied to the gas discharge lamp in the electrode heating state, the control unit turns on the switching element and supplies a predetermined amount of energy to the gas discharge lamp, and, after that, turns off the switching element to charge up the first capacitor and brings the gas discharge lamp to the AC discharging state when a voltage across the first capacitor reaches a predetermined voltage. Preferably, the control unit detects the voltage across the gas discharge lamp when the gas discharge lamp is placed in the AC discharging state so as to control the amount of energy to be supplied to the gas discharge lamp that will be lighted up the next time and that will be placed in the electrode heating state according to the detected voltage. As an alternative, the control unit can detect the voltage across the gas discharge lamp when the gas discharge lamp is placed in the electrode heating state so as to control the amount of energy to be supplied to the gas discharge lamp according to the detected voltage.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.